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            <ol class="chapter"><li><a href="../introduction.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li><a href="../why-rust-and-webassembly.html"><strong aria-hidden="true">2.</strong> Why Rust and WebAssembly?</a></li><li><a href="../background-and-concepts.html"><strong aria-hidden="true">3.</strong> Background And Concepts</a></li><li><ol class="section"><li><a href="../what-is-webassembly.html"><strong aria-hidden="true">3.1.</strong> What is WebAssembly?</a></li><li class="spacer"></li></ol></li><li><a href="../game-of-life/introduction.html"><strong aria-hidden="true">4.</strong> Tutorial</a></li><li><ol class="section"><li><a href="../game-of-life/setup.html"><strong aria-hidden="true">4.1.</strong> Setup</a></li><li><a href="../game-of-life/hello-world.html"><strong aria-hidden="true">4.2.</strong> Hello, World!</a></li><li><a href="../game-of-life/rules.html"><strong aria-hidden="true">4.3.</strong> Rules</a></li><li><a href="../game-of-life/implementing.html"><strong aria-hidden="true">4.4.</strong> Implementing Life</a></li><li><a href="../game-of-life/testing.html"><strong aria-hidden="true">4.5.</strong> Testing Life</a></li><li><a href="../game-of-life/debugging.html" class="active"><strong aria-hidden="true">4.6.</strong> Debugging</a></li><li><a href="../game-of-life/interactivity.html"><strong aria-hidden="true">4.7.</strong> Adding Interactivity</a></li><li><a href="../game-of-life/time-profiling.html"><strong aria-hidden="true">4.8.</strong> Time Profiling</a></li><li><a href="../game-of-life/code-size.html"><strong aria-hidden="true">4.9.</strong> Shrinking .wasm Size</a></li><li><a href="../game-of-life/publishing-to-npm.html"><strong aria-hidden="true">4.10.</strong> Publishing to npm</a></li><li class="spacer"></li></ol></li><li><a href="../reference/index.html"><strong aria-hidden="true">5.</strong> Reference</a></li><li><ol class="section"><li><a href="../reference/crates.html"><strong aria-hidden="true">5.1.</strong> Crates You Should Know</a></li><li><a href="../reference/tools.html"><strong aria-hidden="true">5.2.</strong> Tools You Should Know</a></li><li><a href="../reference/project-templates.html"><strong aria-hidden="true">5.3.</strong> Project Templates</a></li><li><a href="../reference/debugging.html"><strong aria-hidden="true">5.4.</strong> Debugging</a></li><li><a href="../reference/time-profiling.html"><strong aria-hidden="true">5.5.</strong> Time Profiling</a></li><li><a href="../reference/code-size.html"><strong aria-hidden="true">5.6.</strong> Shrinking .wasm Size</a></li><li><a href="../reference/js-ffi.html"><strong aria-hidden="true">5.7.</strong> JavaScript Interoperation</a></li><li><a href="../reference/which-crates-work-with-wasm.html"><strong aria-hidden="true">5.8.</strong> Which Crates Will Work Off-the-Shelf with WebAssembly?</a></li><li><a href="../reference/add-wasm-support-to-crate.html"><strong aria-hidden="true">5.9.</strong> How to Add WebAssembly Support to a General-Purpose Crate</a></li><li><a href="../reference/deploying-to-production.html"><strong aria-hidden="true">5.10.</strong> Deploying Rust and WebAssembly to Production</a></li></ol></li></ol>
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                        <h1 class="menu-title">Rust and WebAssembly</h1> 

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                        <a class="header" href="#debugging" id="debugging"><h1>Debugging</h1></a>
<p>Before we write much more code, we will want to have some debugging tools in our
belt for when things go wrong. Take a moment to review the <a href="../reference/debugging.html">reference page
listing tools and approaches available for debugging Rust-generated
WebAssembly</a>.</p>
<a class="header" href="#enable-logging-for-panics" id="enable-logging-for-panics"><h2>Enable Logging for Panics</h2></a>
<p><a href="../reference/debugging.html#logging-panics">If our code panics, we want informative error messages to appear in the
developer console.</a></p>
<p>Our <code>wasm-pack-template</code> comes with an optional, enabled-by-default dependency
on <a href="https://github.com/rustwasm/console_error_panic_hook">the <code>console_error_panic_hook</code> crate</a> that is configured in
<code>wasm-game-of-life/src/utils.rs</code>. All we need to do is install the hook in an
initialization function or common code path. We can call it inside the
<code>Universe::new</code> constructor in <code>wasm-game-of-life/src/lib.rs</code>:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
pub fn new() -&gt; Universe {
    utils::set_panic_hook();

    // ...
}
#}</code></pre></pre>
<a class="header" href="#add-logging-to-our-game-of-life" id="add-logging-to-our-game-of-life"><h2>Add Logging to our Game of Life</h2></a>
<p>Let's <a href="../reference/debugging.html#logging-with-the-console-apis">use the <code>console.log</code> function via the <code>web-sys</code> crate to add some
logging</a> about each cell in our <code>Universe::tick</code> function.</p>
<p>First, add <code>web-sys</code> as a dependency and enable its <code>&quot;console&quot;</code> feature in
<code>wasm-game-of-life/Cargo.toml</code>:</p>
<pre><code class="language-toml">[dependencies]

# ...

[dependencies.web-sys]
version = &quot;0.3&quot;
features = [
  &quot;console&quot;,
]
</code></pre>
<p>For ergonomics, we'll wrap the <code>console.log</code> function up in a <code>println!</code>-style
macro:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
extern crate web_sys;

// A macro to provide `println!(..)`-style syntax for `console.log` logging.
macro_rules! log {
    ( $( $t:tt )* ) =&gt; {
        web_sys::console::log_1(&amp;format!( $( $t )* ).into());
    }
}
#}</code></pre></pre>
<p>Now, we can start logging messages to the console by inserting calls to <code>log</code> in
Rust code. For example, to log each cell's state, live neighbors count, and next
state, we could modify <code>wasm-game-of-life/src/lib.rs</code> like this:</p>
<pre><code class="language-diff">diff --git a/src/lib.rs b/src/lib.rs
index f757641..a30e107 100755
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -123,6 +122,14 @@ impl Universe {
                 let cell = self.cells[idx];
                 let live_neighbors = self.live_neighbor_count(row, col);

+                log!(
+                    &quot;cell[{}, {}] is initially {:?} and has {} live neighbors&quot;,
+                    row,
+                    col,
+                    cell,
+                    live_neighbors
+                );
+
                 let next_cell = match (cell, live_neighbors) {
                     // Rule 1: Any live cell with fewer than two live neighbours
                     // dies, as if caused by underpopulation.
@@ -140,6 +147,8 @@ impl Universe {
                     (otherwise, _) =&gt; otherwise,
                 };

+                log!(&quot;    it becomes {:?}&quot;, next_cell);
+
                 next[idx] = next_cell;
             }
         }
</code></pre>
<a class="header" href="#using-a-debugger-to-pause-between-each-tick" id="using-a-debugger-to-pause-between-each-tick"><h2>Using a Debugger to Pause Between Each Tick</h2></a>
<p><a href="../reference/debugging.html#using-a-debugger">Browser's stepping debuggers are useful for inspecting the JavaScript that our
Rust-generated WebAssembly interacts
with.</a></p>
<p>For example, we can use the debugger to pause on each iteration of our
<code>renderLoop</code> function by placing <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/debugger">a JavaScript <code>debugger;</code> statement</a>
above our call to <code>universe.tick()</code>.</p>
<pre><code class="language-js">const renderLoop = () =&gt; {
  debugger;
  universe.tick();

  drawGrid();
  drawCells();

  requestAnimationFrame(renderLoop);
};
</code></pre>
<p>This provides us with a convenient checkpoint for inspecting logged messages,
and comparing the currently rendered frame to the previous one.</p>
<p><a href="../images/game-of-life/debugging.png"><img src="../images/game-of-life/debugging.png" alt="Screenshot of debugging the Game of Life" /></a></p>
<a class="header" href="#exercises" id="exercises"><h2>Exercises</h2></a>
<ul>
<li>
<p>Add logging to the <code>tick</code> function that records the row and column of each
cell that transitioned states from live to dead or vice versa.</p>
</li>
<li>
<p>Introduce a <code>panic!()</code> in the <code>Universe::new</code> method. Inspect the panic's
backtrace in your Web browser's JavaScript debugger. Disable debug symbols,
rebuild without the <code>console_error_panic_hook</code> optional dependency, and
inspect the stack trace again. Not as useful is it?</p>
</li>
</ul>

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